One of the best ways of increasing the efficiency of a gas turbine plant is to raise the temperature of the operation cycle. One difficulty met therein is that a high temperature will reduce the useful life of the components and necessitates the use of expensive materials.
There are two ways of reducing above mentioned disadvantage, viz: to arrange air cooling of the components, or to use ceramic, sintered, non-metallic or semi-metallic, highly heat resisting materials.
The parts of the plant being especially exposed to the high temperature is, of course, the combustion chamber, and the first rotor stage as well as the passage means therebetween. These components, therefore, should be made of heat resistant material, and as it is difficult to arrange air cooling of the small and comparatively thin buckets present in the first turbine stage, i.e., usually the compressor driving turbine in an automotive plant, it is further desirable that this turbine stage will rotate at a noticeably lower speed than the following stage, i.e., usually the power turbine. Thus the mechanical stresses in the first named rotor may be kept at a reasonable value, which means a longer working life time without the use of expensive materials.
The heat resistant materials actual for this purpose will here below, for short, be termed ceramic materials, of which silicon nitride and silicon carbide are the most suitable. One important feature to be remembered when using ceramic materials, especially if they are used in direct contact with metallic materials, is the necessity to provide for thermal movements between components of metallic material and components of ceramic material.